The present invention relates to a pneumatic tire and, more particularly, to a pneumatic tire incorporating a puncture sealing layer having improved properties.
There are known various manners of pneumatic tire construction intended to avoid puncture of tires by stones, nails, or other sharp objects. According to one known type of construction, reinforcing means, e.g., so-called breaker layers including metal wire material, for example, which are included in a pneumatic tire made chiefly of rubber or rubber-like material in order to improve resistance of the tire to puncture. However, since from consideration of weight and riding characteristics of the tire, there are practical limits imposed on the size or thickness of such reinforcing means, these means may be unable to prevent particularly long and sharp objects, e.g., nails, from passing therethrough and extending into the inner, air-containing chamber of the tire, which may be constituted by a separate inner tube of extensible rubber material impermeable to air, or by a tire inner wall portion having similar properties, as in the so-called tubeless tire for example. It is therefore also practice to provide a tubeless pneumatic tire with a layer of so-called self-sealing or puncture-sealing material which is very light compared with the weight of the tire as a whole, which is applied on the inner surface of the tire, and, ideally, has properties such that it tends to flow and naturally close a hole formed therein. Conventional composition of such puncture sealing material includes as a principal component polybutene, which is a viscous liquid polymer of low molecular weight, and which, in order to obtain a material having a viscous composition, is mixed with natural rubber or synthetic rubber such as styrene-butadiene copolymer rubber, polybutadiene rubber, butyl rubber, or ethylene-propylene-diene terpolymer rubber. In running tests carried out by the inventors on tubeless tires provided with puncture sealing layers of conventional composition, each tire was pierced with a nail 3.1 mm. in diameter which extended from the tread portion of the tire to and through the puncture sealing layer, tested tires being mounted on automobiles which were then driven for periods of time lasting up to five hours. No leakage of air from nail holes was detected during short runs of one to two hours which were effected immediately after the tires were pierced, but it was found that running the automobiles for five hours at a speed in the range of from 100 to 140 kilometers per hour, i.e., standard cruising speed on a high-speed highway, resulted in nails coming out and a sharp loss of air from tires, or even if nails remained fixed in tires there was leakage of air from around these fixed nails. There was also a sharp loss of air from tires if nails were forcibly removed. Needless to say, both gradual leakage of air and sudden loss of air from a tire can result in bursting of the tire, for example, and can be extremely dangerous.
After concluding the tests, the inventors examined the nail holes formed in the tested tires, and found that, as shown in FIG. 2, puncture sealing material 6' in the immediate vicinity of a nail 9 piercing a tire was pushed radially away from the hole defined by the nail 9, presumably due to movement of the nail 9 during rotation of the tire with the result that instead of the nail 9 being completely enclosed by the sealing material 6', the sealing material 6' defined a hole which was generally centered on the nail 9 and which permitted leakage of air even if the nail 9 remained in the tire. It is evident that as the tire becomes less inflated rotation thereof results in greater movement of the nail 9, with consequent formation of a larger hole. It is thought that such a hole is formed due to a work hardening effect owing to which, after a certain time, the material 6' becomes less plastic and so fails to flow to seal the nail hole.
Another disadvantage of conventional puncture seal material is that it generally has good compatibility with the rubber material constituting the inner lining of the tire, with the result that, particularly in high temperature conditions, the polybutene tends to migrate to and adversely affect the characteristics of the inner tube material.
It is accordingly a principal object of the invention to provide a pneumatic tire having improved puncture sealing characteristics in a wide range of temperature and running conditions.
It is another object of the invention to provide a pneumatic tire wherein there is reduced migration of puncture seal material inner tube material, whereby tire durability is improved.